The present invention relates to novel flame-retardant phosphorus-containing epoxy resins, to a process for their preparation, and to their use. Besides their flame retardancy, the novel phosphorus-containing epoxy resins have especially good storage stability.
Epoxy resins are currently used to prepare molding compositions and coatings with a high level of thermal, mechanical and electronic properties. They are suitable for potting electrical or electronic components and for saturation processes and impregnation processes. The epoxy resins used in electrical engineering are mainly flame-retardant and used for printed circuitboards and insulators.
Epoxy resins for printed circuitboards have hitherto been rendered flame-retardant by including bromine-containing aromatic compounds in the reaction, in particular tetrabromobisphenol A. A disadvantage is that hydrogen bromide is liberated in the event of fire, and this can cause corrosion damage. Under unfavorable conditions, polybrominated dibenzodioxins and furans can also be produced. Considerable technical resources are required for the risk-free disposal of these printed circuitboards by incineration after their use.
The use of aluminum hydroxide is limited by the release of water at higher temperatures. PCT WO 98/31538 describes the use of aluminum hydroxide as flame retardant for NEMA (National Electrical Manufacturers Association) CEM 3 epoxy resin laminates. However, for NEMA FR-4 and FR-5 qualities it has not hitherto been possible to use aluminum hydroxide.
An effective way of rendering epoxy resins flame-retardant is to use organophosphorus compounds. However, compounds which are not bonded by a reaction can migrate out of the laminates and impair the electrical properties. Epoxy resins having chemically bonded phosphorus can be obtained by reacting epoxy resins with organophosphorus compounds having P-OH groups, with addition of the P-OH group to the oxirane ring. This gives phosphorus-modified epoxy resins which remain reactive and can be cured using conventional hardeners.
DE-A 26 52 007 describes the reaction of epoxy resins with phospholanes.
DE-A 43 08 184 describes epoxy resin mixtures made from an aromatic polyamine as hardener and from a phosphorus-modified epoxy resin. This phosphorus-modified epoxy resin is composed of structural units which derive from polyepoxy compounds and from at least one compound selected from the group consisting of phosphinic acids, phosphonic acids, pyrophosphonic acids and phosphonic half-esters.
DE-A 43 08 187 likewise describes epoxy resin mixtures made from an aromatic polyamine as hardener and from a phosphorus-modified epoxy resin. This phosphorus-modified epoxy resin is composed of structural units which derive from polyepoxy compounds and from phosphinic anhydrides and/or phosphonic anhydrides.
DE-A 196 13 066 describes phosphorus-modified epoxy resins made from polyepoxy compounds and from phosphinic or phosphonic acids containing carboxyl groups.
The chemical incorporation of phosphorus compounds into epoxy resins must firstly bring about sufficient flame retardancy and must secondly avoid impairing the mechanical, chemical and electrical properties of the cured resins.
Any reduction in the adhesion of fabrics during prepreg or composite production likewise has to be avoided. Finally, epoxy resin solutions and prepregs for industrial use must have sufficient storage-stability. There is therefore a need for improvement in the provision of epoxy resins with halogen-free flame retardants.
The object of the present invention is to render epoxy resins flame-retardant using phosphorus compounds which are easily obtainable industrially and have a high phosphorus content, the moldings obtained from these epoxy resins after appropriate curing having good mechanical and electrical properties.
The phosphorus-containing epoxy resins, which do not as yet comprise any hardener, are to have adequate storage stability, even in solution. The method of use of the resultant epoxy resins should be similar to that for the materials currently used in industry, i.e. the resins should not be difficult to use.
The object is achieved by means of flame-retardant phosphorus-modified epoxy resins with an epoxy value of from 0.05 to 1.0 mol/100 g containing structural units which derive from
(A) polyepoxy compounds having at least two epoxy groups per molecule, and
(B) organic phosphinic acids.
It is preferable for the organic phosphinic acids to have the formula (I) and for the organic monoalkylated/arylated phosphinic acids to have the formula (II) 
where for formula (I) R1 and R2 are identical or different and each is an alkyl group having from 1 to 10 carbon atoms or an aryl group having from 2 to 10 carbon atoms, and in formula (3) R1 is an alkyl group having from 1 to 10 carbon atoms or an aryl group having from 2 to 10 carbon atoms.
It is preferable for R1 and R2 to be identical or different and for each to be a methyl or ethyl group.
It is preferable for the novel flame-retardant phosphorus-modified epoxy resins to contain, on average, at least one epoxy group.
It is preferable for the novel flame-retardant phosphorus-modified epoxy resins to contain from 1 to 8% by weight of phosphorus.
The object is also achieved by means of a process for preparing flame-retardant phosphorus-modified epoxy resins from epoxy resins and organic phosphinic [or phosphonous] acids as in formula (I) or (II), which comprises reacting the polyepoxy compounds of (A) and the organic phosphinic acids of (B) with one another.
It is preferable for the reaction to take place in a solvent.
It is preferable for polar aprotic solvents to be used, for example N-methylpyrrolidone, dimethylformamide, tetrahydrofuran, dioxane, ethers, such as dialkyl ethers and glycol ethers, ketones, such as methyl ethyl ketone, and/or esters, such as ethyl acetate.
It is preferable for the reaction with the phosphinic acids as in formula (I) to take place at temperatures of from 70 to 100xc2x0 C.
The reaction with the monoalkylated/arylated phosphinic acids as in formula (II) takes place first at temperatures from 80 to 120xc2x0 C. until the acid function has been consumed in the reaction and then continues at temperatures of from 150 to 180xc2x0 C., in the presence of catalysts.
It is preferable for the catalysts used to be phosphonium salts, ammonium salts, metallocenes and/or Lewis acids.
It is preferable for the ratio of equivalents between polyepoxy compound (A) and organic phosphinic acids (B) to be from 1:0.1 to 1:1.
The molar ratio between polyepoxy compound and phosphorus compound may be varied to give reaction products with different phosphorus contents and epoxy values.
The invention further provides the use of the novel flame-retardant phosphorus-modified epoxy resins for producing moldings, composites, coatings or laminates.
As described above, the reaction of the phosphinic acids takes place at temperatures of from 70 to 100xc2x0 C. and can be monitored by measuring the acid number.
The reaction with monoalkylated/arylated phosphinic acids of the formula (II), in contrast, proceeds in two stages. The acid function first reacts with the epoxy group at temperatures of from 80 to 120xc2x0 C. Once this reaction has finished, the temperature is increased to from 150 to 180xc2x0 C. and catalysts added, so that the Pxe2x80x94H group reacts with the epoxide, The reaction can be followed by 31P-NMR spectroscopy. 
FIG. 1: Reaction scheme for the reaction of phosphonous acids with polyepoxy compounds.
It is useful for the monofunctional phosphinic acids of the formula (I) to be reacted with polyepoxy resins of functionality three or above, for example epoxidized novolaks.
The bifunctional monoalkylated/arylated phosphinic acids, in contrast, give better results when reacted with bifunctional epoxy resins.
The phosphorous-modified epoxy resings obtained by the novel process may- as described- be used to produce laminates, composites, molding, or coatings.
Epoxy resins are compounds prepared by a polyaddition reaction between an epoxy resin component and a crosslinking (hardener) component. The epoxy resin component used is an aromatic polyglycidyl ether, such as bisphenol A diglycidyl ether, bisphenol A diglycidyl ether, a polyglycidyl ether of a phenol-formaldehyde resin or of a cresol-formaldehyde resin, a polyglycidyl ester of phthalic, isophthalic, terephthalic or trimellitic acid, an N-glycidyl compound of an aromatic amine or of a heterocyclic nitrogen base, or else a di- or polyglycidyl compound of a polyhydric aliphatic alcohol.
The hardeners used are polyamines such as triethylenetetramine, aminoethylpiperazine and iso-phoronediamine, polyamidoamines, a polybasic acid or anhydride of these, e.g. phthalic anhydride, hexahydrophthalic anhydride, methyl tetrahydrophthalic anhydride, or phenols. The crosslinking may also be brought about by polymerizing with the use of suitable catalysts. The hardener mostly used for printed circuitboards is dicyandiamide.
The fire protection requirement for electrical and electronic equipment is laid down in a variety of specifications and standards for product safety. In the U.S., flame-retardancy testing and approval is undertaken by Underwriters Laboratories (UL). The UL specifications are nowadays accepted worldwide. The fire tests for plastics were developed in order to determine the resistance of the materials to ignition and flame spread. Depending on the fire protection requirements, the materials have to pass horizontal burning tests (Class UL 94 HB) or the more stringent vertical tests (UL 94 V-2, V-1 or V-0). The tests simulate low-energy ignition sources which arise in electrical devices and which can affect plastic parts in electrical assemblies.